Weed flora in arable fields of eastern Ethiopia with emphasis on the occurrence of Parthenium hysterophorus

An exploratory weed survey was carried out on 240 crop fields in eastern Ethiopia to assess the influence of some environmental and crop management factors on weed species composition and distribution, and to investigate the association of the recently introduced Parthenium hysterophorus L. with other components of the weed flora. A total of 102 weed taxa belonging to 36 plant families were recorded. Asteraceae, Poaceae and Fabaceae were the most abundant families based on the number of species recorded. Digitaria abyssinica (Hochst. Ex A. Rich) Stapf was the most frequent species (63%) and Parthenium hysterophorus the second most frequent (54%). The latter species was ranked as the most important weed by 90% of the farmers in the lowlands while 86% of the farmers in the highlands ranked the former species as the worst weed. According to a partial canonical correspondence analysis (pCCA), altitude, rainfall, month of planting, number of weedings and soil type were the major environmental/crop management factors influencing the species distribution in the study area. The first pCCA axis clearly structured highland and lowland weed species while the second axis distinguished those species that grow on sandy soils. Parthenium hysterophorus has, in a very short time period, emerged as one of the most troublesome weed species in eastern Ethiopia.